Abstract
Tandem ene/intramolecular Diels-Alder reaction of ketal 1 and DMAD permits direct entry to strained pentacyclic adducts 2a, b in high yield. Acid-catalysed deprotection and subsequent fragmentation of adduct 2a with BF3.Et2O in dichloromethane stereoselectively affords methanohydroazulene 3.